1. Field of the Invention
This invention relates generally to thermoplastic melt blowing for producing nonwoven plastic textiles. In particular, the invention relates to an apparatus and method for using an array of removable nozzles for discharging molten polymer.
2. Description of the Prior Art
Traditionally, synthetic fibers were, and in some instances still are, produced from thermoplastics extruded through a die that feeds spinnerets. The spinnerets split the molten plastic into thousands of tiny filaments which are then mechanically stretched, cooled and sometimes chemically treated to yield the desired fiber. The plastic fibers can be used to form plastic textiles.
More recently, a new process for forming thermoplastic fibers has been developed known as melt blowing, in which the fibers and subsequent textiles are formed in a simple continuous process. To melt-blow plastic fibers , jetstreams of heated air are placed in close proximity to the plastic filaments exiting from specialized strand plates fed by an extruder. The field of rapidly moving air, exhaust velocity of several thousand feet per second, transforms the plastic filaments into fibers and delivers the airborne fibers to a collection drum or belt where a fibrous web is formed through random mechanical entanglement and heat bonding of the fibers. The distance between the drum and the die plate, as well as other factors well known to those skilled in the art, determine the various characteristics of the fibrous web products and its utility. Various uses include thermal and acoustical insulation, batting for pillows, stuffing for mattresses and comforters, clothing insulation and construction, absorbents for hydrocarbons and chemicals, and wipes.
The field of melt blowing has many patents relating to the die head, the molten plastic orifice, the gas orifice, desired temperatures and velocities, and preferred thermoplastics. One of the early patents in the field, U.S. Pat. No. 3,379,811, issued on Apr. 23, 1968 to Hartmann, describes and claims an apparatus and method for melt blowing molten polymer in which a fluid stream for attenuating the exiting polymer into filaments is provided through two channels and their corresponding orifices located on opposite sides of each polymer discharge orifice.
U.S. Pat. No. 3,441,468, issued on Apr. 29, 1969 to Siggel, describes and claims a method for producing non woven felt-like textiles from melt-blown synthetic polymers by combining a non shrinkable polymer extruded into a stream of hot steam and a shrinkable polymer extruded into a stream of hot gas.
U.S. Pat. No. 3,755,527, issued on Aug. 28, 1973 to Keller, describes and claims a process for melt blowing plastic textiles having a high tear resistance. Molten polymer material is extruded between two knife edge streams of hot gas. Specific temperature, flow rates and viscosity limits, as well as the distance between the discharge orifices and collection drives for a specific filament diameter, are described in the patent.
U.S. Pat. No. 3,825,379, issued on Jul. 23, 1974 to Lohkamp, describes and claims a melt blowing die in which the thermoplastic is discharged through capillary tubes soldered in channels milled in the die. The milled channels are believed to enable alignment of the discharge orifices within tight tolerances and less expensively than is possible with channels that are drilled into the die.
U.S. Pat. No. 3,954,361 describes and claims a melt blowing apparatus in which a die head has multiple thermoplastic flow passages surrounded by channels such that gas flow uniformly encircles the thermoplastic flow passages.
In U.S. Pat. No. 4,380,570, issued on Apr. 19, 1983, to Schwarz, an apparatus and process for melt blowing a thermoplastic product is described and claimed wherein the molten polymer is first passed through a first heating zone at low incremental increases in temperature and then rapidly passed through the discharge nozzles at high incremental increases in temperature.
Additional melt blowing apparatus and methods are disclosed in U.S. Pat. Nos. 3,825,380, 3,849,241, 3,888,610, 3,970,417, and 4,295,809. The foregoing patents are all hereby incorporated by reference as if fully set forth herein.
Despite the many advances made in the field of melt blowing plastics during the last twenty five years, many problems still exist which result in an expensive and inefficient process. For example, the molten plastic discharge channels of melt blowing apparatus are typically machined directly into the die, either drilled into the face of the die, or where the die comprises two or more parts coupled together, milled within one or more of the die parts. Due to the large block of steel necessary to provide the required length over diameter ratio of the discharge channels, the diameter generally on the order of ten to thirty thousands of an inch, the channels are expensive to manufacture and difficult to service. If a particular project calls for a different discharge orifice diameter, a new die has to be cast. Even where a solid block is replaced with nozzles soldered to a strand plate, if a discharge orifice, or its corresponding channel becomes clogged which if left this way will result in a non-uniform and low quality textile, it is extremely difficult and expensive, if at all possible, to clear the clog. The expense is both a result of the cost of repair or replacement and production downtime. This is an especially prevalent problem in the field of recycled plastics where the materials used are replete with impurities.
Accordingly, it is an object of the present invention to provide an efficient and economical method and apparatus for use therewith, for producing melt-blown thermoplastic fibers and non-woven textiles made therefrom. It is also an objective of the present invention to provide high velocity and high volume gas flow uniformly and in close proximity to the discharged molten polymer.
In furtherance of these objectives, the melt blowing apparatus of the present invention comprises an extruder having at one end a die head with one or more openings through which molten plastic is extruded, multiple nozzles each having a shoulder at a back end abutting the openings in the die head for receiving the extruded molten plastic, and a discharge orifice at the front end for discharging the molten plastic into ambient air; a strand plate having an array of nozzle holes at a back end through which the multiple nozzles are inserted; an air chamber defined by the strand plate through which the multiple nozzles pass; and alignment strands for maintaining a desired spatial orientation and alignment of the multiple nozzles.
The long axis of the strand plate of the present invention can be divided into multiple short sections which can be joined to form a single seamless strand plate by forming a seam so as to traverse multiple columns of nozzle passages and providing an additional nozzle passage for each passage in a column lost to the seam.
Ambient air is pumped by any air conveying device such as a compressor into a direct flame chamber in which the air is heated. The heated gas is then channeled, either through the die head and into the air chamber, or directly through the strand plate, into the air chamber.
In accordance with the melt blowing process of the present invention, molten plastic is extruded through a die head and discharged into ambient air through removable nozzles surrounded by high volume, high velocity heated air. Nozzle alignment is maintained by forming an array of alignment strands and placing the nozzles therebetween in tangential contact with the alignment strands. This alignment means also allows for high gas discharge volume and velocity around the nozzles.
A cover plate having an array of holes each with a diameter larger than the outer diameter of each of the multiple nozzles and concentric with and corresponding to the array of multiple nozzles may optionally be placed over the strand plate so that each nozzle passes through a corresponding hole in the cover plate. The cover plate secures the alignment strands in position and also creates an annular path around each nozzle for uniform discharge of the heated air around each nozzle. Alternatively, a retainer plate may be used in lieu of a cover plate to secure the alignment strands and maintain the nozzles in their proper orientation. In this case, the heated air surrounds each nozzle by flowing through the spaces formed between each of the tangential points of contact of the alignment strands and the nozzles. The retainer plate is attached to the strand plate and simply secures the outer perimeter of nozzles or alignment strands, as the case may be, in their desired position.